Highly Stretchable Conductive Covalent Coacervate Gels for Electronic Skin

Highly stretchable electrically conductive hydrogels have been extensively researched in recent years, especially for applications in strain and pressure sensing, electronic skin, and implantable bioelectronic devices. Herein, we present a new cross-linked complex coacervate approach to prepare conductive hydrogels that are both highly stretchable and compressive. The gels involve a complex coacervate between carboxylated nanogels and branched poly(ethylene imine), whereby the latter is covalently cross-linked by poly(ethylene glycol) diglycidyl ether (PEGDGE). Inclusion of graphene nanoplatelets (Gnp) provides electrical conductivity as well as tensile and compressive strain-sensing capability to the hydrogels. We demonstrate that judicious selection of the molecular weight of the PEGDGE cross-linker enables the mechanical properties of these hydrogels to be tuned. Indeed, the gels prepared with a PEGDGE molecular weight of 6000 g/mol defy the general rule that toughness decreases as strength increases. The conductive hydrogels achieve a compressive strength of 25 MPa and a stretchability of up to 1500%. These new gels are both adhesive and conformal. They provide a self-healable electronic circuit, respond rapidly to human motion, and can act as strain-dependent sensors while exhibiting low cytotoxicity. Our new approach to conductive gel preparation is efficient, involves only preformed components, and is scalable.


Legend for Supplementary Movies
Movie S1

Compression sensing
Video showing the response of an LED connected in series with a battery and a cylindrical PEI-(P6.0)0.9-/NG/Gnp3.1 gel cylinder as this is compressed and released.

Tension sensing
Video showing the response of an LED connected in series with a battery and a PEI-(P6.0)0.9-/NG/Gnp3.1 gel film as this is stretched and released

Nanogel characterization
We first synthesized pH-responsive poly(EA-MAA-DVB) NG particles by aqueous emulsion polymerization. 1 TEM studies indicated a number-average diameter, DTEM, of 44 ± 7 nm ( Figure   S1A). Acid titration data ( Figure S1B) indicated that these NG particles contained 64 wt.% MAA and had a pKa of 6.4. According to DLS studies, the intensity-average NG diameter increased from 69 nm to 271 nm when raising the solution pH from 5.8 to 9.8 ( Figure S1C). The NG particles acquire anionic character owing to ionization of the MAA repeat units, with the zeta potential becoming increasingly negative as the solution pH exceeds the pKa ( Figure S1D). A SAXS pattern recorded for a concentrated aqueous dispersion (20 wt.%, Figure S1E) contains a structure peak at a scattering vector (q) of 0.012 Å -1 with a corresponding centre-to-centre distance (L = 2π/q) of 52 nm. The latter value agrees well with the average inter-NG separation of 60 nm calculated from 2 L = Nd 1/3 using the number density of particles (Nd) and a diameter of 44 nm from TEM. All the SAXS data obtained in this study are summarized in Table S1.

PEI-NG complex coacervate gel characterization
A PEI-NG gel was prepared as a control for this study. PEI-NG is a new type of coacervate gel 3 prepared by simply mixing PEI and NG and heating at 50 °C for 20 h (Scheme S1). These gels comprise NGs separated by branched PEI chains, 3 as depicted in Scheme S1. SAXS patterns for the gel contained a broad shoulder in the range q = 0.010 to 0.012 Å -1 corresponding to a mean centreto-centre NG inter-particle separation distance (= 2/q) of 52 to 63 nm ( Figure S2A). The compressive strength of the gel is more than 5.6 x 10 3 kPa ( Figure S2B) and its breaking strain (> 98%) under compression was beyond the upper limit value accessible with our instrument. This physical gel is highly stretchable with a breaking strain of 1045 % and a modulus of 10.5 kPa ( Figure   S2C). Dynamic tensile data ( Figure S2D) showed a resilience (% work of deformation recovered upon strain removal) of 40 -45 % ( Figure S2E). The sacrificial ionic bonds between RNH3 + , R2NH2 + , R3NH + and RCOOgroups as well as the inherent deformability of PEI and NG account for the high breaking strain observed for the PEI-NG gel 3 .  In order to further investigate the mechanism for crosslinking a number of PEI/P6.0 solutions without NGs were prepared and heated at 50 °C for ~ 20 h and then examined using vial inversion for evidence of gelation (see Figure S5A). Interestingly, the PEI/P6.0 solution did not form a gel at the concentration used to prepare the PEI-(P6.0)0.9-NG gel (see Vial E). An image for the latter gel is shown in Figure S5B for comparison. Importantly, when the PEI + P6.0 concentration was increased to ~ 50 wt.% at a similar epoxide-to-primary amine (1°) group ratio a gel formed provided this ratio was greater than or equal to 0.87 (Vial D). (It is assumed that 1/3 rd of the amine groups in PEI are primary amines 4 .) These data provide strong evidence that chemical crosslinking between PEI and PEGDGE occurred as depicted in Scheme S2.   No sediment layers were evident when PEI was added.      Figure S14. Comparison of the initial and equilibrium volume swelling ratios for the gels. The initial swelling ratio corresponds to the as-made state. The pH values of the as-made gels were 8.6 to 9.8.
The equilibrium swelling values were obtained after immersion of the gels in PBS buffer (pH 7.4) for 5 days.